The present invention relates generally to current sensing devices for electrical systems, and more particularly to fault indicators for alternating current power systems that are capable of indicating permanent and temporary faults.
Various types of self-powered fault indicators have been constructed for detecting electrical faults in power distribution systems, including clamp-on type fault indicators, which clamp directly over cables in the systems and derive their operating power from inductive and/or capacitive coupling to the monitored conductor; and test point type fault indicators, which are mounted over test points on cables or associated connectors of the systems and derive their operating power from capacitive coupling to the monitored conductor.
Other prior art fault indicators may be either of the manually resetting type, wherein it is necessary that the indicators be physically reset, or of the self-resetting type, wherein the indicators are reset upon restoration of line current. Examples of such fault indicators are found in products manufactured by E.O. Schweitzer Manufacturing Company of Mundelein, Ill., and in U.S. Pat. Nos. 3,676,740, 3,906,477, 4,063,171, 4,234,847, 4,375,617, 4,438,403, 4,456,873, 4,458,198, 4,495,489, 4,974,329, 5,677,678, 6,016,105, 6,133,723 and 6,133,724.
Detection of fault currents in a monitored conductor by a prior art fault indicator is typically accomplished by magnetic switch means, such as by a calibrated magnetic reed switch, in close proximity to the conductor being monitored. Upon occurrence of an abnormally high fault-associated magnetic field around the conductor, the magnetic switch actuates a trip circuit that produces current flow in a trip winding to position an indicator target or flag visible from the exterior of the indicator to a trip or fault indicating position. Upon restoration of current in the conductor, a reset circuit is actuated to produce current flow in a reset winding to reposition the target indicator to a reset or non-fault indicating position, or the fault indicator may be manually reset.
Rather than a target or flag indicator, some prior art fault indicators utilize one or more light emitting diodes (LEDs) to display a fault condition.
However, fault conditions may be distinguished as temporary faults and permanent faults. Temporary faults are not infrequently caused by tree branches or animals, such as birds or squirrels, coming into contact with conductors. Various other phenomena can also cause a temporary fault. When the temporary fault condition ceases, a reclosing relay in the electrical distribution system is able to reclose and restore power to the affected conductors. Thus, after a temporary fault, the voltage and load current in the conductor may be unchanged. It is likely that no repair of the power system will be required after a temporary fault occurs, but an inspection may be desirable.
On the other hand, a permanent fault usually means that the fault continues long enough that the reclosing relay is unsuccessful in its attempts to restore power to the affected conductors, and it therefore locks out. As a result, the power to the affected conductors is interrupted, and the voltage on the affected conductors drops to zero. Thus, a permanent fault will typically require service to repair the condition that caused the fault before power can be restored to the affected conductors.
There is therefore a need for a fault indicator that can determine whether a temporary fault or a permanent fault has occurred on the monitored conductor.
There is a further need for such a fault indicator that can display both temporary and permanent fault conditions.
The need also arises for a fault indicator that will continue to display temporary or permanent fault conditions for a predetermined amount of time, such as in the range of one hour to twenty-four hours, rather than self-resetting upon restoration of current in the conductor. Such timed reset fault indicators should be capable of self-resetting after termination of the predetermined time.
Some of these applications also require voltage in-rush restraint and/or current in-rush restraint to prevent false tripping, and false activation of the temporary or permanent fault displays, due to voltage and/or current inrush, such as when a reclosing relay of a power distribution system closes.
Because of the compact construction and limited power available in self-powered fault indicators, it is preferable that the desired functions of the fault indicator be accomplished with minimal structure and with internal circuitry that has minimal current drain on a high capacity battery. The fault indicator must also provide highly reliable and extended operation over a number of years.
Accordingly, it is a general object of the present invention to provide a new and improved fault indicator that determines whether a fault condition is temporary or permanent.
Another object of the present invention is to provide a fault indicator that displays both temporary and permanent fault conditions such that service personnel are informed of the type of fault that occurred.
Yet another object of the present invention is to provide a fault indicator that continues to display the temporary or permanent fault condition for a predetermined amount of time, such as in the range of one hour to twenty four hours, and preferably about four hours, and that automatically resets the display upon expiration of the predetermined amount of time.
A further object of the present invention is to provide a fault indicator that initially displays a permanent fault upon the occurrence of a fault in the monitored conductor, that reevaluates the monitored conductor after about one minute, and that continues to display the permanent fault condition if the load current is then below about 5A.
Another object of the present invention is to provide a fault indicator that initially displays a permanent fault upon the occurrence of a fault in the monitored conductor, that reevaluates the monitored conductor after about one minute, and that changes the display to a temporary fault condition if the load current is then greater than about 5A.
A still further object of the present invention is to provide a fault indicator that initially displays a permanent fault when the load current rises above the present trip setting, that reevaluates the monitored conductor after about one minute, that adjusts the trip setting to a high trip setting that is related to the higher load current, and that terminates the permanent fault display at the end of about one minute.